The present invention relates to an optical head for recording signals on, and reproducing signals from, an optical disk, optical components of the optical head, a method of manufacturing the optical head and the components thereof, and an optical disk apparatus.
A data recording/reproducing system using an optical disk as recording medium, such as an optical disk drive, has an optical head. The optical head applies a laser beam to the optical disk to record data on the optical disk and receives a light beam reflected from the disk to read recorded data from the disk. The optical head comprises an optical system, a focusing control means and a tracking control means, which cooperate to apply a laser beam to a correct position in a recording track provided on the optical disk.
In recent years various types of disks have been developed. The most recently developed optical disk is a digital video disk (DVD). The DVD has the same diameter (i.e., 12 cm) as the compact disk (CD) which is an optical disk hitherto known. The DVD comprises two substrates, each having a thickness of 0.6 mm. The DVD is therefore 1.2 mm thick, as thick as the CD. Since signal-recording substrate is thinner than its counterpart of the CD, the accuracy of recording signals on and reproducing them from the substrate is less influenced when the substrate is tilted. The DVD therefore has a recording density much higher than that of the CD. It can record as much information as 4.7 GB (gigabytes).
The tracks formed on the DVD are arranged at intervals of 0.74 .mu.m, about half the track interval of the CD (i.e. , 1.6 .mu.m).To record signals on and reproduce them from the DVD, an optical head is used which emits a laser beam having a wavelength ranging from 640 nm to 670 nm. This wavelength is shorter than the wavelength (i.e., 770 nm to 810 nm) of the laser beam applied to a CD. In the optical system of the optical head, a laser beam must be focused to have a diameter small enough to read data from the DVD. To read data from the CD, a laser beam must be focused to have a diameter adapted to the track interval of the CD.
It is demanded that the optical head for reading signals from a DVD be designed to read signals from a CD as well. In this regard, it should be noted that an optical head for reading data from a DVD must have a light source which emits a laser beam having a wavelength short enough to read data from the DVD and also an optical system which forms a laser beam spot small enough to read data from the DVD.
On the other hand, an optical head for reading signals from a CD must have a light source which emits a more intense laser beam having a wavelength of 780 nm to read/write data from/to a CD-R (a recordable CD) or a CD-RW (a rewritable CD) and also an optical system which forms a beam spot suitable for reading/writing data from/to the CD-R or CD-RW.
An optical head has been designed which can read signals from both a DVD and a CD. The optical head comprises one light source and two optical systems. The light source emits a beam having a waveform ranging from 640 nm to 670 nm. The first optical system focuses the beam , forming a beam spot which suitable for reading data from the DVD, and the second optical system focuses the beam, forming a beam spot suitable for reading data from the CD. The first optical system or the second optical system must be moved to a position where it can focus the beam, in accordance with whether a DVD or a CD is used. Needless to say, the mechanism has many movable parts. Comprising two optical systems and many movable parts, the optical head can hardly be made sufficiently small and may malfunction when vibrated.
Another type of an optical head has been devised which can read signals from both a DVD and a CD. A head of this type is shown in, for example, FIG. 21 of Jpn. Pat. Appln. KOKAI Publication 6-195742. This optical head has two light sources and one optical system. The first light source emits a beam suitable for reading data from a DVD, while the second light source emits a beam suitable for reading data from a CD. It is demanded that the optical head having two optical heads be modified to become smaller and be improved in performance.